CIS 542 Embedded Systems Programming – Summer 2013 Lecture ...

We've already seen how to make sure that it progresses to the right state, but now we also want to
check that it correctly updates the speed.
Obviously, the solution here is testing. Testing is the activity of giving input to the code to see how it
behaves. The goal of testing is not to show that it is correct, but rather to show that there are bugs (or
faults, as we say).
Broadly speaking, there are two ways to create test cases for a piece of code:
• Black-box testing: look at the specification and come up with inputs that exercise as much of
the spec as possible, i.e. try all the different conditions that are described
• White-box testing: look at the code and come up with inputs that exercise as many
statements/paths/branches as possible, i.e. try all the code
Let's start with black-box testing. Consider this part of the spec:
“if the driver chooses to accelerate, the speed increases by 10, unless that would put the speed over
100, in which case it doesn't change”
There are two conditions here that we'd want to check:
1. the driver chooses to accelerate, but that doesn't put the speed over 100
2. the driver chooses to accelerate, and that does put the speed over 100
For each of these, we make the assumption that if it works correctly for some input, it will also work
correctly for “similar” inputs. This is known as equivalence partitioning. For instance, if we decide to
use 20 as the speed for #1, and it works correctly, we probably don't need to also test 21, 22, 30, 40, 80,
etc.
So we can now create some test code like this:
/* this tests accelerating but not going over 100 */
int curr_speed = 20;
enum events event = accel;
enum states curr_state = moving;
int new_speed = control(curr_speed, event, &curr_state);
if (new_speed != 30)
printf(“control returned wrong speed!\n”);
if (curr_state != moving)
printf(“control returned wrong state!\n”);
/* this tests accelerating but trying to go over 100 */
curr_speed = 95;
event = accel;
curr_state = moving;
new_speed = control(curr_speed, event, &curr_state);
if (new_speed != 95)
printf(“control returned wrong speed!\n”);
if (curr_state != moving)
printf(“control returned wrong state!\n”);